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geologic timescale

geologic timescale, a chronological scale of earth's history used to measure the relative or absolute age of any part of geologic time. Of the numerous timescales, the most common is based on geologic time units, which divide time into eras, periods, and epochs. Each division is based on a specific set of geological or paleontological conditions that make it different from the other divisions, such as varying rock type or fossils within the strata. The largest unit is the eon; eons are subdivided into eras; eras into periods; and some, usually more recent periods, into epochs. In some timescales, epochs are further divided into ages. Each geologic time chart varies, depending on the latest findings dating rocks and fossils of that particular age, or on the country where the chart originated. For each unit and its subdivisions, which are listed in the table entitled Geologic Timescale, see separate articles.

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.

More on geologic timescale from Infoplease:

  • Geologic Timescale (table) - Geologic Timescale Era Period Epoch Approximate duration Approximate number of years ago Cenozoic ...
  • Permian period - Permian period Permian period [from Perm, Russia], sixth and last period of the Paleozoic era (see ...
  • period, in geologic time - period period, unit of time on the geologic timescale. Periods are shorter than an era and longer ...
  • epoch - epoch epoch, unit of geologic time that is a subdivision of a period. The Pleistocene and Holocene ...
  • Carboniferous period - Carboniferous period Carboniferous period , fifth period of the Paleozoic era of geologic time (see ...

See more Encyclopedia articles on: Geology and Oceanography

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